Cassettes for holding magnetic recording tape are well known. Typically, such cassettes include a shell which encloses a supply reel of magnetic recording tape and a take-up reel. The free end of the tape on the supply reel is attached to a length of nonmagnetic, usually transparent leader tape, which is threaded over a series of tape guides and is attached to the hub of the take-up reel without additional threading or attachment steps. Recording onto the tape involves inserting the cassette into a recording or playing apparatus which passes the tape over a magnetic head while winding it from the supply reel to the take-up reel. After rewinding the recorded tape onto the supply reel, the recorded material can be played back by following steps similar to the recording steps.
Since cassettes typically contain sufficient tape for more than one recording, it is desirable for a user to know the relative amounts of tape on the supply reel and on the take-up reel. The user might wish to estimate whether sufficient blank tape is present on the supply reel to make a recording of a given length. A known method of estimating the available recording time is to estimate the length of tape on the supply reel by measuring the diameter of the roll of tape on the reel. Based upon the length of the tape on the roll, and the speed at which the recording apparatus transports the tape past the recording head, the available recording time can be calculated.
Measuring tape roll diameter requires using windows and transparent flanges on the tape reels for viewing the tape. Since the user must look through two layers of transparent material, it is important that both sides of both the window and the transparent flange be clean and free of scratches or other defects. Of the four surfaces involved, only the outside of the cassette window is accessible for cleaning.
While suitable transparent polymeric thermoplastic materials are available for producing cassette windows and flanges, these materials suffer from certain disadvantages. These materials tend to be higher in cost and less durable than opaque materials. Furthermore, the number of additives available for improving mechanical properties of transparent thermoplastics is limited by the need to maintain transparency. For example, transparent styrene and styrene-acrylonitrile (SAN) are brittle and easily cracked during assembly and use. Additives used to increase impact strength tend to degrade the transparency of the material to an unacceptable level. Another problem which arises in the molding of transparent parts from thermoplastics is that the flow of the material being molded often leads to variations in optical properties, called flow lines, which produce distortion and detract from the usefulness and appearance of the product.
Known methods for providing more precise estimates of the amount of tape contained on a reel include measuring scales. One scale is disclosed in U.S. Pat. No. 4,501,396, and is attached to a videocassette window. Other tape cassettes provide scale markings molded into the window itself for this purpose. One difficulty with these scales is that they are located a distance from the tape, so that changes in the direction of viewing of the tape can cause significant error, due to parallax, in measuring tape roll diameter.
U.S. Pat. No. 4,846,419 discloses a measuring scale molded directly into the reel flange, to place the scale nearer to the tape, thereby reducing, although not eliminating, error due to parallax. Measuring scales molded into plastic surfaces suffer from an additional problem. Molding imperfections in the region surrounding the scale markings are common, and these imperfections can cause optical distortion which makes reading of the scale difficult and introduces measurement error.
A further disadvantage of transparent tape reels is that the choice of flange color is severely limited. In selling products into a competitive market, the use of attractive colors is an important device for increasing customer appeal.
There is a need for a cassette reel flange having improved mechanical properties, particularly improved impact strength, while still permitting the user to visually estimate the amount of tape contained on the reel. There is also a need for an easily-read tape roll measurement scale which has greatly reduced parallax and optical distortion, to provide greater accuracy in estimating tape length, and which can be transparent, translucent, or opaque.